Materials that ‘remember’ their history have attracted a lot of attention in research, particularly biomedical applications. Shape memory alloys, such as NiTi, have been studied. These materials have limited shape memory deformation capabilities and do not degrade in the body.
In order to improve the shape memory deformation capabilities, shape memory polymers (SMPs) were developed. SMPs belong to a group of polymers which show dual shape properties. As illustrated in FIG. 1, the polymer can be molded in a particular shape A. This polymer can then be deformed to another shape B. ‘Memory’ or recovery of shape A can be elicited by a change in the thermal conditions. Such polymers have been proposed for many applications particularly in bio-applications.
Conventional SMPs utilize a combination of 2 crystallizable or rigid segments, one of which is switchable between hard and soft properties at a certain transition temperature (Ttrans). When the material is heated above Ttrans, only one of the crystalline domains will remain. The polymer shape can then be ‘fixed’ by using the appropriate mold into a shape A. The fixture will be cooled below Ttrans and can be deformed in any manner into shape B. After deformation, at temperatures below Ttrans, the polymer is unable to recover on its own due to the rigidity imposed by the lower melting crystalline segment. However, when the fixture is heated, the lower melting crystalline segment melts and the fixture returns to shape A. Many SMPs rely on crosslinked polymers which give slow response as the slow relaxation process of the recovery gives rise to a lower elasticity entropy upon recovery. This presence of chemical cross-links also limits the processability of the SMPs.
Using these polymers, different shapes can be fashioned and inserted into the body as an implant. Upon exposure to body heat, the polymer can form its original shape and secure itself in the targeted area. The time of response for the recovery of the shape for conventional SMPs takes between 10 seconds to hours. This time scale is too long for applications that require an almost instantaneous response.
Furthermore, the materials used for the development of shape memory polymers include materials such as polyurethanes, crosslinked hydrogels, polynorbornene, poly(styrene-block-butadiene) which are not biodegradable. Such materials may not be biocompatible if the use is desired in humans. SMPs for biomedical applications which undergo good recovery after high strain changes are desired.